Title: Photodetectors
1Chapter 6
2Content
- Physical Principles of Photodiodes
- pin, APD
- Photodetectors characteristics (Quantum
efficiency, Responsivity, S/N) - Noise in Photodetector Circuits
- Photodiode Response Time
- Photodiodes structures
3pin Photodetector
w
The high electric field present in the depletion
region causes photo-generated carriers to
Separate and be collected across the reverse
biased junction. This give rise to a current
Flow in an external circuit, known as
photocurrent.
4Energy-Band diagram for a pin photodiode
5Photocurrent
- Optical power absorbed, in the depletion
region can be written in terms of incident
optical power, - Absorption coefficient strongly depends
on wavelength. The upper wavelength cutoff for
any semiconductor can be determined by its energy
gap as follows - Taking entrance face reflectivity into
consideration, the absorbed power in the width of
depletion region, w, becomes
6-1
6-2
6Optical Absorption Coefficient
7Responsivity
- The primary photocurrent resulting from
absorption is - Quantum Efficiency
- Responsivity
6-3
6-4
6-5
8Responsivity vs. wavelength
9Avalanche Photodiode (APD)
APDs internally multiply the primary photocurrent
before it enters to following circuitry. In
order to carrier multiplication take place, the
photogenerated carriers must traverse along a
high field region. In this region, photogenerated
electrons and holes gain enough energy to ionize
bound electrons in VB upon colliding with them.
This multiplication is known as impact
ionization. The newly created carriers in the
presence of high electric field result in more
ionization called avalanche effect.
Optical radiation
Reach-Through APD structure (RAPD) showing the
electric fields in depletion region and
multiplication region.
10Responsivity of APD
- The multiplication factor (current gain) M for
all carriers generated in the photodiode is
defined as - Where is the average value of the total
multiplied output current is the primary
photocurrent. - The responsivity of APD can be calculated by
considering the current gain as
6-6
6-7
11Current gain (M) vs. Voltage for different
optical wavelengths
12Photodetector Noise S/N
- Detection of weak optical signal requires that
the photodetector and its following amplification
circuitry be optimized for a desired
signal-to-noise ratio. - It is the noise current which determines the
minimum optical power level that can be detected.
This minimum detectable optical power defines the
sensitivity of photodetector. That is the optical
power that generates a photocurrent with the
amplitude equal to that of the total noise
current (S/N1)
13Signal Calculation
- Consider the modulated optical power signal P(t)
falls on the photodetector with the form of - Where s(t) is message electrical signal and m is
modulation index. Therefore the primary
photocurrent is (for pin photodiode M1) - The root mean square signal current is then
6-8
6-9
6-9
6-10
14Noise Sources in Photodetecors
- The principal noises associated with
photodetectors are - 1- Quantum (Shot) noise arises from
statistical nature of the production and
collection of photo-generated electrons upon
optical illumination. It has been shown that the
statistics follow a Poisson process. - 2- Dark current noise is the current that
continues to flow through the bias circuit in the
absence of the light. This is the combination of
bulk dark current, which is due to thermally
generated e and h in the pn junction, and the
surface dark current, due to surface defects,
bias voltage and surface area. - In order to calculate the total noise presented
in photodetector, we should sum up the root mean
square of each noise current by assuming that
those are uncorrelated. - Total photodetector noise currentquantum noise
current bulk dark current noise surface
current noise
15Noise calculation (1)
- Quantum noise current (lower limit on the
sensitivity) - B Bandwidth, F(M) is the noise figure and
generally is - Bulk dark current noise
- is bulk dark current
- Surface dark current noise is the
surface current.
6-11
6-12
Note that for pin photodiode
6-13
16Noise calculation (2)
- The total rms photodetector noise current is
- The thermal noise of amplifier connected to the
photodetector is - input resistance of amplifier, and
is Boltzmann cte.
6-14
6-15
17S/N Calculation
- Having obtained the signal and total noise, the
signal-to-noise-ratio can be written as - Since the noise figure F(M) increases with M,
there always exists an optimum value of M that
maximizes the S/N. For sinusoidally modulated
signal with m1 and
6-16
6-17
18Photodetector Response Time
- The response time of a photodetector with its
output circuit depends mainly on the following
three factors - 1- The transit time of the photocarriers in
the depletion region. The transit time
depends on the carrier drift velocity and
the depletion layer width w, and is given by - 2- Diffusion time of photocarriers outside
depletion region. - 3- RC time constant of the circuit. The
circuit after the photodetector acts like RC low
pass filter with a passband given by
6-18
6-19
19Photodiode response to optical pulse
Typical response time of the photodiode that is
not fully depleted
20Various optical responses of photodetectors
Trade-off between quantum efficiency response
time
- To achieve a high quantum efficiency, the
depletion layer width must be larger than - (the inverse of the absorption
coefficient), so that most of the light will be
absorbed. At the same time with large width, the
capacitance is small and RC time constant getting
smaller, leading to faster response, but wide
width results in larger transit time in the
depletion region. Therefore there is a trade-off
between width and QE. It is shown that the best
is
21Structures for InGaAs APDs
- Separate-absorption-and multiplication (SAM) APD
- InGaAs APD superlattice structure (The
multiplication region is composed of several
layers of InAlGaAs quantum wells separated by
InAlAs barrier layers.
light
22Temperature effect on avalanche gain
23Comparison of photodetectors